Recently, solar cells developed as next-generation energy source are being rapidly spread for household and industrial purposes.
In general, a solar cell module includes a plurality of solar cells. The solar cells are filled in a filling sheet and fixed by the filling sheet and a back sheet is attached at least on a lower surface for the purpose of sealing, and thereby the solar cell module is prepared. FIG. 1 schematically shows the configuration of an existing general solar cell module.
Referring to FIG. 1, a solar cell module generally has a structure wherein a reinforced glass 3 onto which light is incident, an upper filling sheet 2a, solar cells C, a lower filling sheet 2b and a back sheet 1 are stacked sequentially. Between the upper filling sheet 2a and the lower filling sheet 2b, the plurality of solar cells C are filled and fixed while connected electrically. As for the upper/lower filling sheet 2a, 2b, an ethylene vinyl acetate (EVA) sheet which is advantageous in fixing the solar cells C is used. The back sheet 1 is bonded to the filling sheet 2b and protects the solar cells C.
A long lifetime with no deterioration of output over a long period of time is required for a solar cell module. To ensure the long lifetime, it is important to isolate moisture or oxygen that may negatively affects the solar cell C and to prevent deterioration of the back sheet 1 due to UV, etc. Also, low cost of the back sheet 1 is strongly demanded by consumers recently. Accordingly, the back sheet 1 has to be made of a material with good heat resistance, durability, weather resistance, etc. so that it can endure high temperature, humidity, UV, etc. well in order to ensure long lifetime of the solar cell module, and needs to be available at low cost.
The back sheet 1 for a solar cell module generally has a structure in which a heat-resistant polyethylene terephthalate (PET) film as a base film is stacked with a weather-resistant fluorine-based film. That is to say, as shown in FIG. 1, the back sheet 1 generally includes a PET film 1a and a fluorine-based film 1b formed on the PET film 1a. The fluorine-based film 1b is attached to upper and lower surfaces of the PET film 1a by an adhesive.
The PET film 1a is useful as a base film of the back sheet 1 because it is superior not only in heat resistance but also in durability including mechanical strength. A polyvinylidene fluoride (PVDF, Polyvinylidene fluoride) film is frequently used as the fluorine-based film 1b. The PVDF film exhibits excellence weather resistance, etc.
Heat is generated when the solar cell is operated. The generated heat needs to be dissipated (discharged) to outside as much as possible to ensure good power generation (i.e., photoconversion efficiency) of the solar cell. Although the existing back sheet 1 is superior in heat resistance, durability, weather resistance, etc., power generation (photoconversion efficiency) of the solar cell needs to be improved because it has no or unsatisfactory heat-dissipating property of dissipating the heat generated in the solar cell.
Also, as described above, the existing back sheet 1 is prepared by attaching the fluorine-based PVDF film 1b on the PET film 1a using an adhesive to improve weather resistance, etc. However, because the PVDF film 1b is expensive, it is not easy to lower the cost of the back sheet 1.
In addition, the back sheet 1 needs to be firmly attached to the lower filling sheet 2b, i.e. the EVA sheet, of the solar cell module to ensure sealing. However, the existing back sheet 1 is problematic in that the adhesive force between the fluorine-based PVDF film 1b and the EVA sheet 2b is weak.